Compositions based on ethylene polymers and containing carbon black have been used, extensively, in the production of conductive shields about electrical cables. Conductive shields are essential components of cable design and construction serving, in electrical power cables, as conductive and insulation shields and also providing protection against short circuits.
Ethylene polymer compositions, from which conductive shields are produced, are prepared by admixing an ethylene polymer, conductive carbon black and other components such as an antioxidant and, if the compositions are to be crosslinked, an organic peroxide. A key parameter with respect to such compositions is the degree of dispersion of the carbon black in the polymer matrix. The degree of carbon black dispersion directly controls the homogeniety of the compositions which in turn affects the physical and electrical properties thereof. Also, when the compositions are used to form conductive shields about electrical cables, good dispersion of the carbon black is essential for long term cable reliability. Carbon black agglomerates tend to form protrusions at the interface between the insulation and the conductive shield of the electrical cable. These protrusions become points of electrical stress leading to cable failure.
The problem of carbon black dispersibility has been accentuated in recent years due to the increased use of carbon black having a high surface area, generally in excess of about 500 m.sup.2 /gram, in formulating compositions to be used in the extrusion of conductive shields about cables. It has been found that reduced amounts of carbon black having a high surface area can be used to provide equivalent product conductivities, compared to compositions containing standard carbon blacks. Compositions, therefore, have been formulated containing increased amounts of polymer which enhance certain properties of the resultant compositions.
Despite improvement in properties, the successful commercial utilizaton of these carbon blacks has been hampered by processing difficulties, specifically poor dispersibility in the polymer matrix. As a result, it has been necessary to utilize extensive compounding procedures and/or special equipment in order to insure that high surface area carbon black is adequately dispersed in the polymer matrix.